In an effort to meet stringent federal government emissions standards, engine systems may be configured with exhaust gas recirculation (EGR) systems wherein at least a portion of the exhaust gas is recirculated to the engine intake. The EGR systems may include low-pressure EGR systems that recirculate exhaust gas from downstream of an exhaust turbine to upstream of an exhaust compressor, as well as high-pressure EGR systems that recirculate exhaust gas from upstream of an exhaust turbine to downstream of an exhaust compressor. Such EGR systems enable an increase in engine dilution, decrease in exhaust emissions, and improvements in fuel economy.
However the inventors herein have recognized potential issues with such systems. As one example, there may be limits to the amount of residuals that can be tolerated in the engine due to combustion stability constraints. For example, due to engine combustion stability limits at the prevalent engine operating conditions, even though further engine dilution is possible (e.g., by further opening an EGR valve), an amount of residuals actually delivered to the engine may be limited (e.g., at high loads) so as to deliver the demanded torque.
Conversely, there may be conditions where further dilution is required but is not deliverable due to a limit of the EGR system. For example, if further dilution is required (e.g., for knock mitigation at high engine loads) when one or more of an EGR valve of the high-pressure EGR system and the low-pressure EGR system is fully opened (that is, at an upper opening limit), a further increase in dilution may not be provided. Likewise, if dilution is required to be reduced when one or more of an EGR valve of the high-pressure EGR system and the low-pressure EGR system is fully closed (that is, at a lower closing limit), a further decrease in dilution may not be provided. In each case, engine performance is degraded due to a limit (combustion stability limit or valve opening/closing limit) being reached.
The inventors herein have recognized that engine dilution issues may be overcome in a boosted engine system that uses a binary flow turbine. In one example, managing of residuals may be improved by a method comprising adjusting a scroll valve coupled to an inlet of an outer scroll of a multi-scroll exhaust turbine responsive to engine dilution. By adjusting the scroll valve based on engine dilution, exhaust residuals may be managed even when a combustion stability limit is approached, and/or an opening or closing limit of an EGR valve is approached. For high pressure EGR (HP-EGR) systems, closing the scroll valve can provide higher exhaust manifold pressure so that exhaust manifold pressure is above intake manifold pressure, thereby allowing HP-EGR to flow.
In one example, in response to an engine dilution request being higher than an engine dilution that can be provided by the EGR system (such as when a high-pressure EGR valve or a low-pressure EGR valve has reached a fully open position), the engine dilution request may be provided by decreasing the opening of the scroll valve. Wastegate adjustments may be coordinated with the scroll valve adjustment to further improve managing of the exhaust residuals. Alternatively, if combustion stability limits are reached, the scroll valve may be moved to a more open position to quickly reduce residuals by lowering internal and/or HP EGR.
As such, while the scroll valve is adjusted based on the engine dilution request, the EGR valves may be maintained open. Alternatively, their opening may be decreased while the scroll valve is adjusted, thereby further extending their limits. In still other examples, while the scroll valve is adjusted, each of an LP-EGR valve and a HP-EGR valve may be adjusted to vary a ratio of LP-EGR to HP-EGR while providing the engine dilution.
In this way, scroll valve adjustments in a binary flow turbine can be advantageously used to manage engine dilution and residual delivery. By closing the valve when the opening limit of an EGR valve is reached, further engine dilution may be provided. By extending the range of engine operating conditions over which exhaust gas can be recirculated, engine dilution benefits are maximized, improving engine performance and fuel economy.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.